The abundance of HNCO and its use as a diagnostic of environment

TL;DR

This study models the chemistry of HNCO in star-forming regions, revealing its formation pathways, the variability of the HNCO/CS ratio as a diagnostic tool, and challenging previous assumptions about UV influence.

Contribution

It introduces a comprehensive chemical model of hot cores that accounts for grain surface reactions and challenges prior interpretations of the HNCO/CS ratio as a UV field indicator.

Findings

HNCO formation on grain surfaces is necessary to match observed abundances.

The HNCO/CS ratio varies with time and initial sulphur abundance in hot cores.

The ratio is not solely determined by the ambient UV field, contrary to previous hypotheses.

Abstract

We aim to investigate the chemistry and gas phase abundance of HNCO and the variation of the HNCO/CS abundance ratio as a diagnostic of the physics and chemistry in regions of massive star formation. A numerical-chemical model has been developed which self-consistently follows the chemical evolution of a hot core. The model comprises of two distinct stages. An initial collapse phase is immediately followed by an increase in temperature which represents the switch on of a central massive star and the subsequent evolution of the chemistry in a hot, dense gas cloud (the hot core). During the collapse phase, gas species are allowed to accrete on to grain surfaces where they can participate in further reactions. During the hot core phase surface species thermally desorb back in to the ambient gas and further chemical evolution takes place. For comparison, the chemical network was also used to model a simple dark cloud and photodissociation regions. Our investigation reveals that HNCO is inefficiently formed when only gas-phase formation pathways are considered in the chemical network with reaction rates consistent with existing laboratory data. Using currently measured gas phase reaction rates, obtaining the observed HNCO abundances requires its formation on grain surfaces. However our model shows that the gas phase HNCO in hot cores is not a simple direct product of the evaporation of grain mantles. We also show that the HNCO/CS abundance ratio varies as a function of time in hot cores and can match the range of values observed. This ratio is not unambiguously related to the ambient UV field as been suggested - our results are inconsistent with the hypothesis of Martin et al (2008). In addition, our results show that this ratio is extremely sensitive to the initial sulphur abundance.